TY  - JOUR
A1  - El Mellah, Ileyk
A1  - Sander, Andreas Alexander Christoph
A1  - Sundqvist, Jon Olof
A1  - Keppens, Rony
T1  - Formation of wind-captured disks in supergiant X-ray binaries Consequences for Vela X-1 and Cygnus X-1
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. In supergiant X-ray binaries (SgXB), a compact object captures a fraction of the wind of an O/B supergiant on a close orbit. Proxies exist to evaluate the efficiency of mass and angular momentum accretion, but they depend so dramatically on the wind speed that given the current uncertainties, they only set loose constraints. Furthermore, these proxies often bypass the impact of orbital and shock effects on the flow structure. Aims. We study the wind dynamics and angular momentum gained as the flow is accreted. We identify the conditions for the formation of a disk-like structure around the accretor and the observational consequences for SgXB. Methods. We used recent results on the wind launching mechanism to compute 3D streamlines, accounting for the gravitational and X-ray ionizing influence of the compact companion on the wind. Once the flow enters the Roche lobe of the accretor, we solved the hydrodynamics equations with cooling. Results. A shocked region forms around the accretor as the flow is beamed. For wind speeds on the order of the orbital speed, the shock is highly asymmetric compared to the axisymmetric bow shock obtained for a purely planar homogeneous flow. With net radiative cooling, the flow always circularizes for sufficiently low wind speeds. Conclusions. Although the donor star does not fill its Roche lobe, the wind can be significantly beamed and bent by the orbital effects. The net angular momentum of the accreted flow is then sufficient to form a persistent disk-like structure. This mechanism could explain the proposed limited outer extension of the accretion disk in Cygnus X-1 and suggests the presence of a disk at the outer rim of the neutron star magnetosphere in Vela X-1 and has dramatic consequences on the spinning up of the accretor.
KW  - accretion, accretion disks
KW  - X-rays: binaries
KW  - stars: black holes
KW  - stars: neutron
KW  - supergiants
KW  - stars: winds, outflows
Y1  - 2019
U6  - https://doi.org/10.1051/0004-6361/201834498
SN  - 1432-0746
VL  - 622
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Grinberg, Victoria
A1  - Hell, Natalie
A1  - El Mellah, Ileyk
A1  - Neilsen, Joseph
A1  - Sander, Andreas Alexander Christoph
A1  - Leutenegger, Maurice
A1  - Fürst, Felix
A1  - Huenemoerder, David P.
A1  - Kretschmar, Peter
A1  - Kuehnel, Matthias
A1  - Martinez-Nunez, Silvia
A1  - Niu, Shu
A1  - Pottschmidt, Katja
A1  - Schulz, Norbert S.
A1  - Wilms, Joern
A1  - Nowak, Michael A.
T1  - The clumpy absorber in the high-mass X-ray binary Vela X-1
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase similar to 0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium, and neon whose strengths and presence depend on the overall level of absorption. These features imply a co-existence of cool and hot gas phases in the system, which we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.
KW  - X-rays: individuals: Vela X-1
KW  - X-rays: binaries
KW  - stars: winds, outflows
KW  - stars: massive
Y1  - 2017
U6  - https://doi.org/10.1051/0004-6361/201731843
SN  - 1432-0746
VL  - 608
PB  - EDP Sciences
CY  - Les Ulis
ER  -